Joists or trusses include at least two chord or chord-like members connected by suitable load distributing lattices or webs. A typical joist or truss consists of two parallel chord members joined by an intermediate supporting web which may comprise a solid section or a pattern of struts designed to ensure that such structure has requisite load bearing capabilities.
A typical web of the type under consideration takes the form of a suitable structural steel rod bent upon itself into an undulating or zig-zag configuration of generally uniform pattern with the respective apices presented by the bent rod secured or connected to the spaced upper and lower chord members which members are usually arranged in parallel relation.
When such structure is fabricated out of steel and used to support the flooring or carry ceiling components it is commonly referred to as an "open-web" steel joist.
A typical method of attaching or connecting chord members to the web apices is by means of electrical resistance welding.
Electrical resistance welding is a process whereby fusion is produced by the heat generated from the resistance of the metal components to be joined together to the flow of electrical current when contacted and clamped between appropriate electrodes within an electrical circuit. No external heat source is required in order to achieve fusion, nor are any fluxes or filler materials necessary.
The current for resistance welding is normally supplied through a transformer which transforms the high voltage, low amperage power supply to usable high amperage current at low voltages. The pressure for the electrode clamping forces is generated by a suitable hydraulic system.
In any electrical conductor, current flow creates heat. The amount of heat (H) generated depends upon the amount of current (I), the resistance of the conductor (R) and the time (T) during which the current is flowing. Heat generated in resistance welding can be expressed in the following manner: EQU H=I.sup.2 RT
H=heat generated in joules;
I=current in rms amps;
R=resistance of the work in ohms;
T=time of current flow in seconds.
This formula indicates that the heat generated is directly proportional to the square of the welding current and to the resistance and to the time of current flow. The heat generated in such a procedure is used in part to fuse the chord and web members together in the regions of contact but with some of their useful heat being lost by conduction to the work pieces themselves and to the contacting clamping electrodes.
Radiation loss to the atmosphere is relatively insignficant.
The heat lost by conduction from the weld zone to the work pieces and to the contacting electrodes is directly proportional to the temperature differential existing between them. The greater the temperature differential therefore the greater the heat loss. This heat loss not only produces discoloration, warpage or twisting but undesirable metallurgical changes in the work pieces in the form of large grain growth in the region immediately surrounding the region of contact commonly designated the weld zone or the "nugget".
Large grain growth is undesirable because it produces weaknesses and brittleness at the weld joints and cause failure.
It follows that by controlling heat lost or dissipation by conduction, discoloration, warpage or twisting and large grain growth can be reduced. Further it is apparent that by lowering the temperature differential the greater will be the tendency to localize or confine the generated heat to the weld zone or "nugget", and therefore the escape of said generated heat energy from said region of contact will be minimized.
According to the formula, resistance is a critical factor in the generation of heat.
In making a weld, the current is passed from one electrode through the base metal in the work pieces to the other electrode. During this passage the current encounters several resistance zones including:
(a) the electrical resistance of each electrode; PA0 (b) the contact resistance between each electrode and the base metal, the magnitude of which depends on the surface condition of the base metal and electrode, the size and contour of the electrode face, and the electrode clamping force; PA0 (c) the resistance of each piece of the base metal which is directly proportional to the resistivity of the base metal and its thickness and inversely proportional to the cross-sectional area of the current path; PA0 (d) the base metal interface where the weld formation starts, which is the point of highest resistance and therefore the point of greatest heat generation.
Should the surface of the base metal present oxides or scale then the resistance of the base metal would be correspondingly higher than if the base metal was free of same. Therefore the heat generated in the weld zone would be higher than normal and concomitantly the temperature differential between the weld zone and the surrounding metal would be greater than normal which results in undesirable grain growth as mentioned above. As well, the scale tends to impede the conduction of heat between the two work pieces being joined, and presents an unclean surface for welding which results in sparking during the welding operation and ultimately produces an unacceptable joint or connection. Accordingly, each work piece of base metal should be kept substantially free of scale.
Also, for best results the electrodes should be kept substantially free of the oxide coatings.
In making a satisfactory weld there is one factor not always given due consideration because of the difficulty in accurately predicting the precise effect. This factor is correct heat balance which is the condition in which the fusion zone in each work piece to be joined undergoes approximately the same degree of heating. The quality of the weld is enhanced when each work piece to be joined experiences the same degree of heating.
As mentioned above the resistance of each work piece is inter alia inversely proportional to the cross-sectional area of the current path. Therefore when resistance welding two equal cross-sectional areas of the same metal the heat balance is generally automatic. However, joists or trusses typically have disproportionate thicknesses or cross-sectional areas of chord members and web rods and therefore the heat balance is not automatic as the cross-sectional area in the common region of contact heat up at different rates. In welding dissimilar cross-sectional areas, a longer period of current flow or heat cycle is required to provide a more uniform distribution of heat throughout the asymmetrical resistance path extending between the clamping electrodes.
A number of pieces of equipment and proposals have heretofore been developed and used for welding the chord components of a joist or a truss to the web on an assembly line basis so as to reduce the cost of same. The quality however of the welds of the finished joists or trusses has also given some concern.
U.S. Pat. No. 3,158,731 discloses apparatus for automatically fabricating a truss where the chords are continuously formed in roller dies from coiled strip material while web wire also from a coil is straightened then bent into a zig-zag shape whereupon all three components are assembled together then resistance welded together and finally the truss so produced is automatically cut into lengths and painted.
Another alternative is illustrated in U.S. Pat. No. 3,288,977 which relates to a welding device which automatically moves the latticing strips in a step-by-step manner, with pauses between the motion for profiling the latticing into a zig-zag configuration and for welding the latticing to the chords. This patent further discloses testing equipment which comprises hydraulic cylinders and grippers which will apply a force F to one of the welded joints on the upper chord, while a force F/2 is applied to each of the adjacent two welded joints on the lower chord.
Still another alternative is disclosed by U.S. Pat. No. 3,427,699 which relates to a production line operation wherein chords of predetermined cross-section are stacked at two separate stations and in parallel relation and individually fed forward from the bottom of the stack, then sheared to length and aligned with a web of zig-zag configuration. The chords and web are then fed forwardly together in continuous alignment and welded together.
A further alternative is illustrated in U.S. Pat. No. 3,487,861 which discloses apparatus providing means for performing the steps of simultaneously supplying a pair of straight side wires and an intermediate straight wire from several sources, in side-by-side relation in one dircection along a path bending the intermediate wire into zig-zag form while driven along such path and then welding the apices to the flanking side wires to complete the truss structure.
Still another arrangement is found in U.S. Pat. No. 3,641,303 which illustrates a method and apparatus for continuously producing a truss element in successive unit lengths by shaping an extended length of suitable strip material to present a zig-zag configuration, while in a position adjacent a chord members provided with integral ribs formed thereon, then welding the web and chord together. Each chord presents ribs of relatively small cross-sectional area which flow under the force and heat generated by the clamping electrodes of a suitable resistance welding mechine, and thereby form the welds which join the web to the chord member.